Mandibular plane angle changes with or without premolar extraction treatment in adult orthodontics measured using 3-D cone beam technology

The purpose of this study was to determine whether extraction of four premolars in orthodontic treatment results in closing of the mandibular plane angle, also called the 'wedge effect'. Method: Using a new program, OnDemand3D(TM), pre-treatment and post-treatment 3D cone beam CT scans of 54 patients at the University of Southern California's School of Dentistry were superimposed to compare changes during orthodontic treatment. Using 3D scans to measure changes eliminates distortion effects from traditional lateral cephalograms, landmark identification errors, and superimposition errors. The two 3D scans were superimposed initially on the cranial base. Viewing the image in the sagittal plane, horizontal and vertical changes at the most anterior and most posterior point on the mandible, respectively, were measured in millimeters. Changes at the incisal edge of the central incisor were also measured horizontally and vertically. Then an image slice was made through the arch form to measure horizontal and vertical changes of the maxillary left and right first molar. The two images were then superimposed on the anterior mandibular body. Using similar views, horizontal and vertical measurements were made for the lower central incisor and lower left and right first molar. Results: There was no significant difference (p>0.05) in mandibular movement with extraction of four first premolars and non-extraction orthodontic treatment. In the extraction group, there was a significant (p<0.05) lingual movement of the upper and lower incisors as well as significant (p<0.05) mesial movement of the upper and lower molars compared to the non-extraction group. The mandible showed no significant (p>0.05) difference between the extraction and the non-extraction group. There was a correlation (p>0.05) in lingual movement of the incisors and horizontal movement of the mandible as well as vertical displacement of the incisors and vertical movement of the mandible. Vertical movement of the molars was correlated (p<0.05) with vertical movement of the mandible. Horizontal movement of the upper and lower molars were correlated (p<0.05) as well as vertical movement of the upper and lower molars. Conclusion: There is no 'wedge effect' when extracting four premolars in orthodontic treatment. Vertical movement of the molars should be controlled as there is an increase in vertical facial dimension with extrusion of the molars during mesialization for extraction treatment.